It is well known that static electrical charge generation in close proximity to electronic assemblies may cause damage when the charge is dissipated through the sensitive electronic components that are typically included in an electronic assembly. The problem of electrostatic discharge (ESD) is frequently encountered during the manufacture, maintenance or repair of electronic assemblies when humans are handling the electronic assemblies. Specifically, the human body tends to generate a static electrical charge through a process known as tribocharging. Tribocharging is a phenomenon that results in the gradual accumulation of a static electrical charge on a body, including a human body, after frictional contact with other surfaces. Providing a means to alert an individual of the presence or build-up of a static electrical charge on that individual's body may prevent an accidental or uncontrolled ESD, and may also thus avoid damage to sensitive electronic equipment.
As electronic assemblies continue to be developed with components having increasing sensitivity and population density, the danger of experiencing component damage caused by ESD increases, even with low levels of static discharge. It is thus becoming more and more important to prevent unwanted ESD. Preventing such damage during the manufacture and handling of sensitive electronic assemblies requires that an environment be provided which is essentially free of static electrical charge.
To avoid the generation of static electrical charge in the workplace environment, consideration must be given to the phenomenon of tribocharging described above. A variety of methods may be used to detect and/or remove static charge from tribocharged bodies. One common method for removing static charge from individual workers is an electrically conducting tether connecting the charged body to electrical ground. Typically, the tether is connected between a device such as a conductive wrist band, which makes intimate contact with the individual's skin, and a predetermined electrical ground site. Static electrical charge that an individual worker develops will leak to electrical ground through the wrist band and conductive tether, thereby removing the static electrical charges to provide a safe environment. For example, U.S. Pat. No. 4,398,277 shows a grounding tether, which may also be described as a grounding cord or cable or strap.
While conductive tethers or grounding cords provide effective removal of static electrical charge, they also have the problem of limiting the range of movement of workers who are grounded for safety. They may also interfere with the work the individual is performing. Grounding methods that permit greater freedom of movement have thus also been developed and include special conductive footwear and heel grounders that are be used in combination with conductive flooring.
It is desirable to be able to monitor the effectiveness of various grounding methods. For example, effective operation of passive grounding cords, as previously described, requires intimate electrical contact between the tether and the body where the static electrical charge exists. Any disruption of electrical contact prevents the static electrical charge from being conducted to ground. This possibility of disruption necessitates provision of a means to determine the efficacy of the grounding cord, using a suitable form of monitor. Currently, devices are available that continually monitor any breaks in the grounding circuit. U.S. Pat. Nos. 4,639,825 and 5,057,965, for example, show open circuit monitors that provide an audible warning when the grounding tether ceases to produce a path to ground. These open circuit monitors function only when a tribocharged body is continuously attached to ground. Therefor, other grounding means, including conductive footwear and heel grounders as mentioned above, cannot use open circuit monitoring devices to monitor their effectiveness.
For instances in which the charged body is not continuously attached to ground, other types of monitors must be used. Fixed electric field meters of various types, revealed in, e.g. U.S. Pat. No. 4,562,429, have been used to watch for the presence of dangerous static electrical charge buildup in a work area. As an individual enters into the work area, fixed electric field meters can be used to establish if that individual is electrically charged to such an extent as to represent a hazard to the operations being conducted in the work area. Fixed electric field meters, while in operation, also require that there be a connection from the meter to electrical ground to establish a voltage reference. Therefor, a fixed field meter is not readily mobile and can only monitor a specific area around the meter. Unfortunately, a fixed field meter also has a limited coverage area and has no ability to distinguish the source of potentially damaging charge from among a number of possible sources. Fixed field meters thus have a limited usefulness in many applications, such as when several individuals are working in the same area or when the individuals are moving in areas beyond the coverage area of the meter.
The availability of a static electrical charge warning or monitoring device without an associated cord or tether connected to electrical ground would allow an electronics assembler to move freely around a work location. U.S. Pat. No. 5,461,369 describes a device that is worn by the user and does not require a grounding tether. The device of the '369 patent records the occurrence of an ESD event. The device includes a display element to provide immediate indication of a change in voltage associated with discharge of static electricity. One embodiment of the discharge indicator provides a device to be worn on a person's wrist, similar in size and appearance to a wristwatch. Another form of device appears as a ring to be worn on a person's finger. A third embodiment of the discharge indicator may be clipped to the wearer's clothing. In this case, a connecting wire must pass through the wearer's clothing to make direct contact with the wearer's skin. Although the device of the '369 patent allows a user to move freely without a grounding tether, each embodiment has the disadvantage of requiring direct contact between the device and the wearer. Without direct contact, there will be no record of an ESD event. Also, the device of the '369 patent only alerts a wearer after an ESD event has occurred. The device does not provide the ability to avoid the ESD and its accompanying damage.
As noted above, the monitoring devices such as those disclosed in U.S. Pat. Nos. 5,461,369 and 4,562,429 offer freedom of movement for users. However, they have limited ability to distinguish between an individual and a group of people or they only provide evidence of static electrical discharge after the discharge event occurs and are ill suited to prevent its occurrence. It would clearly be desirable to provide a device which can be worn by the user, and which does not need to be electrically connected to ground, thus providing the user freedom to move about the workplace. It would also be desirable to couple this mobility with the ability to warn a user of an impending ESD event, such that action may be taken to avoid the ESD. With suitable warning, a worker could take action to prevent the impending static electrical discharge from ever occurring, and thereby also prevent any damage that may be caused by such a discharge. Further, it would be desirable if the monitor only needs to be in close proximity of the user, without requiring direct electrical contact with the user.